The present invention relates to the interface between a rack and a ball nut and the lateral movement of the ball nut relative to the rack.
Ball nut drive assemblies are widely used in the steering systems of motor vehicles and are generally configured to be installed around a steering member and driven by an electric motor. The ball nut drive assembly is typically housed in a cast iron casing and typically comprises a ball nut rotatably positioned intermediate a bearing thrust adjuster and a bearing annulus on a ball screw that forms a portion of the steering member. The rotational motion of the ball nut via the electric motor causes the ball screw to laterally translate the ball nut. The lateral motion of the ball screw is imparted to the steering rod and to steerable wheels rotatably connected to the ends of the steering rod to alter the position of the wheels relative to the vehicle body, thereby steering the vehicle.
One of the more common problems associated with ball nut drive assemblies involves axial play or backlash between the ball nut and the ball screw. This backlash, which is a function of the manufacturing tolerances associated with the ball nut drive assembly components, causes uneven wear, noise, and poor response when either the ball screw or the ball nut begin traveling axially along the other. Since the manufacturing tolerances are generally critical to the smooth translation of the ball nut along the ball screw, the bearing thrust adjuster is sufficiently dimensioned and preloaded to bias the ball nut against the bearing annulus, thereby preventing any free play in the lateral direction along the steering member.
The present invention contemplates the replacement of the conventional bearing thrust adjuster, thrust adjuster lock nut, and bearing annulus with a spring compliant member. By replacing the afore-mentioned items with the spring compliant member, the need for a separate bearing pre-load arrangement on the ball nut is eliminated; therefore, the need for periodic adjustments to the afore-mentioned items to compensate for backlash between the ball nut and the ball screw is also eliminated. Furthermore, the substitution of the conventional bearing thrust adjuster, thrust adjuster lock nut, and bearing annulus with the spring compliant member facilitates the removal and reinstallation of the ball nut in order to replace the belt that drives the ball nut in the event that the belt breaks or becomes worn.
The inventive ball nut drive assembly includes a ball nut having an aperture extending axially therethrough, a ball screw extending through the aperture of the ball nut and being cooperatively engaged with the ball nut, and a spring compliant member received on the ball screw and positioned adjacent the ball nut. The spring compliant member is tensioned to bias the ball nut in the axial direction along the ball screw. The ball nut is positioned within a housing, preferably fabricated of aluminum, and is generally rotatably supported therein by first and second sets of ball bearings. The spring compliant member is in intimate contact with a first inner surface of the housing and biases the ball nut against a second inner surface of the housing. The first and second inner surfaces may be races that encapsulate the first and second sets of bearings. The spring compliant member may be a spring wave washer, a Belleville washer, or some other type of spring compliant device.
The above-mentioned invention provides various benefits to the use of a ball nut drive assembly. In particular, the reduction in the number of parts and the simplification of the remaining parts reduces the cost of equipment required to manufacture the components. Furthermore, the cost of service on the ball nut drive assembly is reduced in that a reduction in the number of parts lessens the likelihood that the assembly will require service over its lifetime. Moreover, utilizing aluminum in the design, as opposed to cast iron as in the prior art, reduces the cost of the raw material involved in the manufacture of the ball nut drive assembly.